My 1984 chevy with a 700r4 had this type of problem for a while. Took a look at it and over time the valve inside the trans gets stuck and sometimes it shifts really funny. Mine was shifting every 10 MPH which was random. I just stopped on the throttle and held it wide open until i reached about 70 MPH and did that over and over, and it stopped doing it. Could give it a try but no promises.